Conventionally, there has been a moving body detector described in Japanese Published Unexamined Patent Application No. 2009-281890. The conventional moving body detector in the above-mentioned document, as shown in FIG. 2, includes a first oscillation circuit 1a, a second oscillation circuit 1b, a frequency selection circuit 2, a transmitter 3, a receiver 4, a phase detection circuit 5, a low pass filter 6, a comparator 7, an operational circuit 8a, a threshold circuit 8b, and a control circuit 9. The first oscillation circuit 1a is configured to oscillate a transmission signal having a frequency fa, and the second oscillation circuit 1b is configured to oscillate a reference signal having a frequency fb that is different from the frequency of the transmission signal. The transmitter 3 is configured to transmit ultrasonic waves to a space to be monitored, when receiving the transmission signal outputted from the first oscillation circuit 1a. The receiver 4 is configured to output a receiving signal, when receiving a reflected wave generated due to reflection of the ultrasonic waves by an object (a moving body) 50 that exists within the space to be monitored. The phase detection circuit 5 is configured to mix the receiving signal and any one of the transmission signal and the reference signal, to obtain a Doppler signal according to a frequency difference between the receiving signal and the any one. The frequency selection circuit 2 is configured to switch the transmission signal and the reference signal inputted to the phase detection circuit 5. The low pass filter 6 is configured to remove unnecessary high frequency components from the Doppler signal outputted from the phase detection circuit 5. The comparator 7 is configured to compare the Doppler signal outputted from the low pass filter 6, with a predetermined reference value, to convert into a binary signal. The operational circuit 8a is configured to perform an operational processing of the binary signal outputted from the comparator 7. The threshold circuit 8b is configured to detect the moving body 50 within the space to be monitored, based on an operation result by the operational circuit 8a, to output a detection signal. The control circuit 9 is configured to have a moving body detection mode of detecting the moving body 50 and a malfunction detection mode of detecting whether or not the detector is normally operating. In the moving body detection mode, the control circuit 9 is configured to control the frequency selection circuit 2 to input the transmission signal to the phase detection circuit 5. In the malfunction detection mode, the control circuit 9 is configured to control the frequency selection circuit 2 to input the reference signal to the phase detection circuit 5.
In the case where the control circuit 9 operates in the moving body detection mode, the transmitter 3 transmits, to the space to be monitored, the ultrasonic waves having the same frequency as the transmission signal, when receiving the transmission signal having the frequency fa from the first oscillation circuit 1a. Then, the receiver 4 receives the reflected wave having a frequency f′a generated due to reflection of the ultrasonic waves by the moving body 50 that exists within the space to be monitored and then outputs, to the phase detection circuit 5, the receiving signal having the same frequency as the reflected wave. The phase detection circuit 5 mixes the receiving signal, and the transmission signal that is outputted from the first oscillation circuit 1a via the frequency selection circuit 2, and then outputs the Doppler signal according to the frequency difference between those signals. Then, the low pass filter 6 removes unnecessary high frequency components from the Doppler signal outputted from the phase detection circuit 5, and then the comparator 7 converts the Doppler signal into the binary signal to output to the operational circuit 8a. The operational circuit 8a performs the operational processing of the binary signal to obtain a moving speed of the moving body 50. Then, when the obtained moving speed exceeds a prescribed value, the threshold circuit 8b determines that the moving body 50 exists within the space to be monitored, and then outputs the detection signal.
Here, when, in the moving body detection mode, the moving body 50 does not exist within the space to be monitored or remains stationary within the space to be monitored, the frequencies of the receiving signal and the transmission signal mixed by the phase detection circuit 5 are equal to each other, and accordingly, it is impossible to obtain the Doppler signal. So, in the conventional moving body detector, to detect whether or not the detector is normally operating, the control circuit 9 operates in the malfunction detection mode and controls the frequency selection circuit 2 to input, to the phase detection circuit 5, the reference signal having the frequency fb that is different from that of the transmission signal. Accordingly, the phase detection circuit 5 mixes the signals having the frequencies different from each other, thereby obtaining a pseudo Doppler signal. Therefore, it is possible to detect whether or not the detector is normally operating, through comparing a result of an operational processing of the pseudo Doppler signal performed by the operational circuit 8a, with a predetermined threshold. That is, it is possible to determine that the detector is normally operating when the result of the operational processing of the pseudo Doppler signal agrees with the predetermined threshold, and that there is a malfunction such as disconnection in any of circuits constituting the detector when the result thereof disagree with the predetermined threshold.
In this conventional moving body detector, the first oscillation circuit 1a, second oscillation circuit 1b, frequency selection circuit 2, operational circuit 8a, threshold circuit 8b and control circuit 9 are configured by a microcomputer. The microcomputer executes a program, thereby achieving the functions of the above-mentioned circuits. Therefore, it is possible to realize the first oscillation circuit 1a, second oscillation circuit 1b and frequency selection circuit 2 through utilizing a timer function provided in the microcomputer, and realize the above-mentioned malfunction detection function with only changing software without seriously changing hardware.
However, it is required that the conventional moving body detector includes two oscillation circuits 1a, 1b, of which the oscillation frequencies are different from each other, in order to realize the malfunction detection function. In addition, if the oscillation circuits 1a, 1b and the like are configured by the microcomputer, two of plural timers in the microcomputer may be occupied to output two signals (the transmission signal and reference signal) of which the frequencies are different from each other.
As explained above, because the conventional moving body detector requires a plurality of pieces of hardware (the oscillation circuits 1a and 1b, or two or more timers of microcomputer) having the same functions, there is a possibility that the cost or the size of the detector is increased with addition of the malfunction detection function.